The present application is directed to a contact chatter detector for use during seismic or vibration testing of electrical devices having contacts, and more particularly to a testing device which monitors the contacts of one or more relays regardless of the state of the relays (i.e., energized, non-energized, or transition) and which detects contact chatter having a duration greater than a predetermined minimim.
Many mechanical switching-type devices employ metal contacts which move toward or away from each other to close or open circuits, respectively. The switching performance of such devices, particularly as they age, may deteriorate and result in contact chatter when the devices are subjected to mechanical forces such as vibration. Relays are among the mechanical-type switching devices that are vibration sensitive. A single-pole, double-throw relay, for example, is illustrated in FIG. 1A and includes two fixed contacts 12 and 14 that are spaced apart by a gap. A movable contact 16 is disposed in the gap and is spring-biased toward contact 12. This is deemed the "normally closed" contact. The relay also includes a solenoid 18 which, when energized, pulls movable contact 16 from contact 12 to contact 14, the "normally open" contact. Vibration may momentarily jolt movable contact 16 away from normally closed contact 12 when solenoid 18 is in the de-energized state or, similarly, may momentarily jolt movable contact 16 away from normally open contact 14 when solenoid 18 is in the energized state. Moreover, during the transition state, as movable contact 16 snaps from one fixed contact to the other, vibration exacerbates the "contact bounce" phenomenon as movable contact 16 hits a fixed contact and rebounds from it. The same problems, of course, occur in double-pole, triple-pole, etc. relays, which have more than one pair of fixed contacts and more than one movable contact.
In practice relays are frequently fabricated and shipped in lots with the characteristics of the relays being fairly uniform within a lot. Accordingly, a representative sample of the relays in a lot, such as three randomly selected relays, may be tested in order to evaluate the entire lot. For example IEEE 501-1978 "IEEE Standard, Seismic Testing of Relays" provides that relays must be tested while in the energized mode, the de-energized mode, and the transition mode, and that a minimum of three relays must be tested for each mode. The standard requires that all relay contacts are to be continuously monitored for contact chatter during the test, and that chatter of two milliseconds or more must be detectable. Furthermore the standard specifies that 125 VDC must be applied to all contacts. This latter requirement serves to ensure that contact chatter is not missed due to oxidized contacts.
With these testing criteria in mind one might seek to employ the circuit of FIG. 1B to test 4PDT (four pole, double throw) relay 20, which is mounted on vibration mechanism 22. Relay 20 includes normally closed contacts 24, 26, 28, and 30, and normally open contacts 32, 34, 36 and 38. Relay 20 also includes solenoid 40, which is mechanically coupled to the four movable contacts as indicated by dotted arrow 42. It will be apparent that the movable contacts are not specifically illustrated in the schematic diagram of relay 20 but are, instead, present conceptually as parts of the normally closed and normally open contacts. For example normally closed contact 24 represents a fixed contact which touches a movable contact when solenoid 40 is de-energized, while normally open contact 32 represents the adjacent fixed contact to which the movable contact moves when solenoid 40 is energized.
The negative terminal of 125 volt battery 44 is connected to ground and one lead of solenoid 40. The other lead of solenoid 40 is connected to the positive terminal of battery 44 through switch 46. Normally closed contacts 24-30 are connected sequentially between battery 44 and resistor 48 by wires 50, 52, 54, 56, and 58. Resistor 60 and chart recorder 62 are connected parallel to one another between resistor 48 and ground. Normally open contacts 32-38 are connected together by wires 64, 68, and 70, which are connected through resistor 72 to resistor 74 and chart recorder 76. The reason for resistors 78 and 80 will be explained momentarily.
With continuing reference to FIG. 1B, during a test of the de-energized mode switch 46 is open as illustrated and vibration mechanism 22 is activated for perhaps 20 seconds, the mechanism exerting a maximum force of perhaps 44 spectrum gravities on the movable contacts. If all of the normally closed contacts 24-30 remain closed, the full voltage of battery 44 is applied across resistors 48 and 60, resulting in a uniform line produced by recorder 62. Should one or more of contacts 24-30 open momentarily, however, additional resistance (78 or 80 or both) would be inserted into the circuit, so that the signal detected by recorder 62 would drop momentarily. Similarly, during the test the signal received by recorder 76 would be zero unless one or more of normally open contacts 32-38 closes momentarily.
Resistors 78 and 80 might be omitted except for an anomalous situation which would result if contacts 24 and 30 were to bounce open, with the movable contact associated with contact 30 going further and closing contact 38. The closure of contact 38 would not be detected by recorder 76 were resistors 78 and 80 replaced by insulators. Although the closure of contact 38 in this situation may seem unlikely, the response of aged relays during seismic stresses is not well researched, so that all volt combinations should be tested.
With continuing reference to FIG. 1B, relay 20 is energized by closing switch 46, whereupon normally closed contacts 24-30 are opened and normally open contacts 32-38 are closed. In order to connect the now-closed contacts 32-38 in series between resistor 48 and battery 44, however, wire 68 would have to be removed. Furthermore a wire joining wires 52 and 56 would be needed to connect the now-opened contacts 24-30 in parallel for sensing by recorder 76. The transition mode is tested, in accordance with IEEE 501-1978, by activating switch 46 while relay 20 is wired for testing in the energized mode; bouncing is detected if recorders 62 and 76 achieve their expected readings for the energized mode after switch 46 is closed and thereafter deviate from these expected readings.
The circuit of FIG. 1B has several shortcomings. If only three relays from each lot are to be devoted to testing, separate tests and the attendant wiring changes are needed to account for all three relay operating modes. Furthermore the records produced by chart recorders 62 and 76 must be inspected to determine whether the duration of a pulse exceeds a predetermined minimum, such as 2 milliseconds. The inspection problem is complicated because commercially available chart recorders typically have limited response to short duration pulses.